Various automatic landing systems have become available during the past several years, often being incorporated in the aircraft autopilot or flight control system. Basically, these automatic landing systems are digital signal processors that execute control laws that cause the aircraft to follow glide slope and localizer signals so that the aircraft approaches the destination runway without manual control by the pilot. When the aircraft reaches a suitable point in its descent (e.g., aircraft landing altitude of 45 or 50 feet), the automatic landing system initiates a flare maneuver to arrest aircraft rate of descent (sink rate) so that the aircraft touches down on the runway at a proper position and at a suitable sink rate.
In general, prior art automatic landing systems have utilized the aircraft radio altimeter signal to measure landing gear altitude and, hence, initiate the flare maneuver during an automatic landing procedure. In addition, most of these automatic landing systems have utilized the aircraft radio altimeter signal throughout the automatic landing flare maneuver and maneuvering of the aircraft to a safe touchdown on the runway. Using a predetermined flare height initiation altitude (e.g., landing gear altitude of 45 or 50 feet) and the aircraft radio altimeter signal to measure landing gear altitude from the point of flare initiation to touchdown has resulted in certain disadvantages.
First, when the automatic landing flare maneuver is initiated at a fixed altitude of 45 or 50 feet, the aircraft typically begins the flare maneuver at a distance of 100 to 200 feet from runway threshold, with the exact distance in primary part being determined by the location of the glide slope transmitter and the glide slope beam angle for the landing facility at which the automatic landing is being executed. In some cases, however, initiation of the flare maneuver at a 45 or 50 foot aircraft gear altitude causes the aircraft to begin executing the flare maneuver on the order of 500 to 700 feet ahead of the runway threshold. Since the signal supplied by the radio altimeter system in effect follows the terrain along the approach path, irregular terrain near the end of the runway sometimes causes the landing system to unnecessarily execute a substantial amount of pitch attitude adjustments and control column activity prior to the time that the aircraft touches down on the runway. This relatively high degree of pitch attitude and control column activity can cause undue concern on the part of the aircraft pilot. In some cases, the pilot may unnecessarily assume manual control of the aircraft, even when the landing is being executed under low visibility conditions. Further, terrain induced changes in the aircraft radio altimeter signal may result in a relatively high aircraft sink rate at touchdown (i.e., a relatively hard landing). Such a hard landing may result in an undue degree of passenger concern, apprehension and discomfort. Even further, terrain induced variation in the radio altimeter signal can result in a longer than nominal touchdown distance (distance between the point of touchdown and runway threshold). In most situations, the disadvantages have been somewhat troublesome but have been such that the automatic landing system still met all safety and performance requirements. However, in some cases certain automatic landing system equipped aircraft have been prohibited from making automatic landings at certain landing facilities that have irregular landing approach terrain.
Various attempts have been made to at least partially alleviate the above-discussed disadvantages that are encountered when an automatic landing is made at a landing facility that has irregular approach terrain. For example, complementary filtering techniques have been used in which the aircraft radio altimeter signal is in effect combined with vertical acceleration signals produced by the aircraft inertial reference system to generate an aircraft gear altitude signal that provides improved system performance. As is known in the art, basically such complementary filtering constitutes low pass filtering of the radio altimeter signal and high pass filtering of the inertial vertical acceleration signal with the content of the filter output signal that is attributable to the radio altimeter signal and the inertial vertical acceleration signal being determined by the complementary filter frequency.
Prior attempts to use complementary filtering to provide improved automatic landing in the presence of terrain considerations have not been entirely successful. The basic reason is that the complementary filter frequency must be selected to compromise system performance that results under adverse approach terrain relative to system performance under other approach conditions. For example, many runways exhibit a relatively substantial slope that may be on the order of 2.degree.-3.degree.. Moreover, some runways exhibit considerable slope variation, initially sloping up or down and then sloping in the opposite direction. Such runway slope and slope variation induces gear height altitude changes as the aircraft executes the final phase of the automatic landing procedure. If the slope induced changes in altitude are not reflected in the aircraft gear altitude signal, the automatic landing may result in a relatively hard landing, may produce relatively high touchdown dispersion, and may exert substantial control column activity and/or substantial pitch attitude activity during the final phase of the landing procedure. Because of the compromise that must be made to accommodate both approach path terrain and runway slope, prior attempts to provide a gear altitude signal by complementary filtering of the radio altimeter signal and vertical acceleration signal have not resulted in the desired degree of improved system performance.